System and method for a mems device

ABSTRACT

Light in the visible spectrum is modulated using an array of modulation elements, and control circuitry connected to the array for controlling each of the modulation elements independently, each of the modulation elements having a surface which is caused to exhibit a predetermined impedance characteristic to particular frequencies of light. The amplitude of light delivered by each of the modulation elements is controlled independently by pulse code modulation. Each modulation element has a deformable portion held under tensile stress, and the control circuitry controls the deformation of the deformable portion. Each deformable element has a deformation mechanism and an optical portion, the deformation mechanism and the optical portion independently imparting to the element respectively a controlled deformation characteristic and a controlled modulation characteristic. The deformable modulation element may be a non-metal. The elements are made by forming a sandwich of two layers and a sacrificial layer between them, the sacrificial layer having a thickness related to the final cavity dimension, and using water or an oxygen based plasma to remove the sacrificial layer.

This is a continuation in part of U.S. patent application Ser. No.08/032,711, filed Mar. 17, 1993.

BACKGROUND

This invention relates to visible spectrum (including ultra-violet andinfrared) modulator arrays.

Visible spectrum modulator arrays, such as backlit LCD computer screens,have arrays of electro-optical elements corresponding to pixels. Eachelement may be electronically controlled to alter light which is aimedto pass through the element. By controlling all of the elements of thearray, black and white or, using appropriate elements, color images maybe displayed. Non-backlit LCD arrays have similar properties but work onreflected light. These and other types of visible spectrum modulatorarrays have a wide variety of other uses.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features modulation of light inthe visible spectrum using an array of modulation elements, and controlcircuitry connected to the array for controlling each of the modulationelements independently, each of the modulation elements having a surfacewhich is caused to exhibit a predetermined impedance characteristic toparticular frequencies of light.

Implementations of the invention may include the following features. Thesurface may include antennas configured to interact with selectedfrequencies of light, or the surface may be a surface of an interferencecavity. The impedance characteristic may be reflection of particularfrequencies of light, or transmission of particular frequencies oflight. Each of the modulation elements may be an interference cavitythat is deformable to alter the cavity dimension. The interferencecavity may include a pair of cavity walls (e.g., mirrors) separated by acavity dimension. One of the mirrors may be a broadband mirror and theother of the mirrors may be a narrow band mirror. Or both of the mirrorsmay be narrow band mirrors, or both of the mirrors may be broad band,non-metallic mirrors. The cavity may have a cavity dimension thatrenders the cavity resonant with respect to light of the frequencydefined by the spectral characteristics of the mirrors and intrinsiccavity spacing in an undeformed state. One of the mirrors may be ahybrid filter. One (or both) of the walls may be a dielectric material,a metallic material, or a composite dielectric/metallic material. Thecavity may be deformable by virtue of a wall that is under tensilestress. The control circuitry may be connected for analog control of theimpedance to light of each element. The analog control may be control ofthe degree of deformity of the deformable wall of the cavity.

The predetermined impedance characteristic may include reflection ofincident electromagnetic radiation in the visible spectrum, e.g., theproportion of incident electromagnetic radiation of a given frequencyband that is, on average, reflected by each of the modulation elements.The modulation element may be responsive to a particular electricalcondition to occupy either a state of higher reflectivity or a state oflower reflectivity, and the control circuitry may generate a stream ofpulses having a duty cycle corresponding to the proportion of incidentradiation that is reflected and places the modulation element in thehigher state of reflectivity during each the pulse and in the lowerstate of reflectivity in the intervals between the pulses. Thecharacteristic may include emission of electromagnetic radiation in thevisible spectrum. The characteristic may include the amount ofelectromagnetic radiation in the visible spectrum that is emitted, onaverage, by the antennas. The characteristic may be incidentelectromagnetic radiation in the visible spectrum. The modulationelements may include three sub-elements each associated with one ofthree colors of the visible spectrum. The modulation element may beresponsive to a particular electrical condition to occupy either a stateof higher transmissivity or a state of lower transmissivity, and thecontrol circuitry may generate a stream of pulses having a duty cyclecorresponding to the proportion of incident radiation that istransmitted and places the modulation element in the higher state oftransmissivity during each the pulse and in the lower state oftransmissivity in the intervals between the pulses. The characteristicmay include the proportion of incident electromagnetic radiation of agiven frequency band that is, on average, transmitted by each of themodulation elements.

The visible spectrum may include ultraviolet frequencies, or infraredfrequencies.

In general, in another aspect of the invention, the control circuitrymay be connected to the array for controlling the amplitude of lightdelivered by each of the modulation elements independently by pulse codemodulation.

In general, in another aspect, the invention features a modulationelement having a deformable portion held under tensile stress, andcontrol circuitry connected to control the deformation of the deformableportion.

Implementations of the invention may include the following features. Themodulation element may be self-supporting. or held on separate supports.The deformable portion may be a rectangular membrane supported along twoopposite edges by supports which are orthogonal to the membrane. Thedeformable portion, under one mode of control by the control circuitry,may be collapsed onto a wall of the cavity. The control circuitrycontrols the deformable portion by signals applied to the modulationelement, and the deformation of the control portion may be subject tohysteresis with respect to signals applied by the control circuitry.

In general, in another aspect, the invention features modulating lightin the visible spectrum using a deformable modulation element having adeformation mechanism and an optical portion, the deformation mechanismand the optical portion independently imparting to the elementrespectively a controlled deformation characteristic and a controlledmodulation characteristic.

Implementations of the invention may include the following features. Thedeformation mechanism may be a flexible membrane held in tensile stress,and the optical portion may be formed on the flexible membrane. Theoptical portion may be a mirror. The mirror may have a narrow band, or abroad band, or include a hybrid filter.

In general, in another aspect, the invention broadly features anon-metal deformable modulation element.

In general, in another aspect, the invention features a process formaking cavity-type modulation elements by forming a sandwich of twolayers and a sacrificial layer between them, the sacrificial layerhaving a thickness related to the final cavity dimension, and usingwater or an oxygen based plasma to remove the sacrificial layer.

Among the advantages of the invention are the following.

Very high-resolution, full-color images are produced using relativelylittle power. The embodiment which senses the image incident on thearray has relatively low noise. Their color response characteristics aretunable by selection of the dimensions of the antennas. The antenna orcavity embodiments are useful in portable, low power, full colordisplays, especially under high ambient light conditions. Phasecontrolled reflective embodiments are useful in passive light scanningsuch as optical disk readers without moving parts. The emissiveembodiments also could be used as display devices especially inlow-ambient-light conditions.

Because of the dielectric materials used in some embodiments, thedevices have the advantage of being extremely light efficient, makingthem especially appropriate for high intensity projection displays, andreducing or eliminating the need for backlighting in low ambient lightapplications. In addition, more accurate color representations arepossible, as well as designs optimized for the IR and UV. Mechanicalhysteresis precludes the need for active drivers, and this coupled withtheir geometric simplicity and monolithic nature brings defect lossesdown significantly. The devices are also exceptionally fast, low power,and non-polarizing. The fact that they can be reflective and/ortransmissive enhances their flexibility.

The process for fabrication as represented in some embodiments relies onbenign chemicals, minimizing waste disposal problems, and facilitatingthe fabrication of devices on a variety of substrates (e.g., plastics orintegrated circuits) using a larger variety of materials. Devices onplastic substrates have the potential of being extremely inexpensive.All of the manufacturing technologies used are mature, further reducingmanufacturing costs.

Other advantages and features of the invention will become apparent fromthe following description and from the claims.

DESCRIPTION

FIG. 1 is a perspective view of a display device.

FIG. 2 is a perspective schematic exploded view of a representativeportion of the screen of FIG. 1.

FIG. 3 is an enlarged top view of a tri-dipole of FIG. 2.

FIG. 4 is a schematic view of a single dipole antenna of FIG. 3.

FIG. 5 is a schematic perspective view, broken away, of a portion of thescreen of FIG. 1.

FIG. 6 is an enlarged top view of an individual tri-bus of FIG. 2.

FIG. 7 is an enlarged perspective view of a representative portion ofthe screen of FIG. 1.

FIG. 8 is a cross-sectional view along 8-8 of FIG. 7.

FIG. 9 is a diagram of a portion of a control circuit of FIG. 2, and acorresponding dipole antenna of FIG. 3.

FIGS. 10A, 10B, 10C are representative graphs of the input voltage tothe bias source of FIG. 9.

FIG. 11 is a diagram of portions of the control modules for a row ofpixels,

FIG. 12 is a circuit diagram of an oscillator.

FIG. 13 is a schematic diagram of a circuit module of FIG. 2, acorresponding dipole antenna of FIG. 3, and a graphical representationof the output of a binary counter.

FIG. 14 is a circuit diagram of the pulse counter of FIG. 13.

FIGS. 15, 16, 17, 18, and 19 are top views of alternative dipolearrangements.

FIGS. 20A through 20D are perspective views of a cavity device.

FIGS. 21A and 21B are side views of the cavity device.

FIGS. 22A through 22F are graphs of frequency responses of the cavitydevice in different states.

FIGS. 23A and 23B are top and cutaway side views, respectively, of adisplay.

FIGS. 23C and 23D are top and cutaway side views, respectively, ofanother display.

FIG. 24A is a graph of an electromechanical response of the cavitydevice.

FIG. 24B is a graph of an addressing and modulation scheme for adisplay.

FIGS. 25A through 25N and FIGS. 26A through 26K are perspective views ofthe device during assembly.

FIGS. 27A through 27C are side views of dielectric mirrors.

FIG. 27D is a top view of a dielectric mirror.

FIGS. 28A, 28B are perspective and top views of a linear tunable filter.

FIGS. 29A, 29B are perspective and top views of a deformable mirror.

Referring to FIG. 1, device 20 includes a screen 22 for displaying orsensing a high resolution color image (or a succession of color images)under control of power and control circuitry 26. The image is made up ofa densely packed rectangular array of tiny individual picture elements(pixels) each having a specific hue and brightness corresponding to thepart of the image represented by the pixel. The pixel density of theimage depends on the fabrication process used but could be on the orderof 100,000 pixels per square centimeter.

Referring to FIG. 2, each pixel is generated by one so-called tri-dipole30. The boundary of each tri-dipole is T-shaped. The tri-dipoles arearranged in rows 32 in an interlocking fashion with the “Ts” ofalternating tri-dipoles oriented in one direction and the “Ts” ofintervening tri-dipoles along the same row oriented in the oppositedirection. The rows together form a two-dimensional rectangular array oftri-dipoles (corresponding to the array of pixels) that are arranged ona first, external layer 34 of screen 22. The array may be called anelectrically alterable optical planar array, or a visible spectrummodulator array.

On a second, internal layer 36 of screen 22 so-called tri-busses 38(shown as T-shaped blocks in FIG. 2) are arranged in an interlockingtwo-dimensional array 40 corresponding to the layout of the tri-dipoleson layer 34 above. Each tri-dipole 30 is connected to its correspondingtri-bus 38 by a multi-conductor link 42 running from layer 34 to layer36 in a manner described below.

On a third, base layer 44 of screen 22 a set of circuit modules 46 arearranged in a two-dimensional rectangular array corresponding to thelayouts of the tri-dipoles and tri-busses. Each circuit module 46 isconnected to its corresponding tri-bus 38 by a six-conductor link 48running from layer 36 to layer 44 in a manner described below.

Each circuit module 46 electronically controls the opticalcharacteristics of all of the antennas of its corresponding tri-dipole30 to generate the corresponding pixel of the image on screen 22.Circuit modules 46 are connected, via conductors 50 running along layer44, to an edge of layer 44. Wires 52 connect the conductors 50 tocontrol and power circuitry 26 which coordinates all of the circuitmodules 46 to 25 generate the entire image.

Referring to FIG. 3, each tri-dipole 30 has three dipole sections 60,62, 64. The center points 59, 61, 63 of the three sections are arrangedat 120 degree intervals about a point 65 at the center of tri-dipole 30.Each section 60, 62, 64 consists of a column of dipole antennas 66, 68,70, respectively, only ten dipole antennas are shown in each section inFIG. 3, but the number could be larger or smaller and would depend on,e.g., the density with which control circuits 46 can be fabricated, thetradeoff between bandwidth and gain implied by the spacing of theantennas, and the resistive losses of the conductors that connect theantennas to the control circuit 46. Only the two arms of each dipoleantenna are exposed on layer 34, as shown in FIG. 3. The dipole antennasof a given section all have the same dimensions corresponding to aparticular resonant wavelength (color) assigned to that section. Theresonant wavelengths for the three sections 60, 62, 64 are respectively0.45 microns (blue), 0.53 microns (green), and 0.6 microns (red).

Referring to FIG. 4, each dipole antenna 80 schematically includes twoLs 82, 84 respectively made up of bases 86, 88, and arms 90, 92. Thebases of each antenna 80 are electrically connected to the correspondingcircuit module 46. The span (X) of arms 90, 92 is determined by thedesired resonant wavelength of dipole antenna 80; for example, for aresonant wavelength of lambda, X would be lambda/2. Dipole antennas 66,68, 70 have X dimensions of 0.225 microns (lambda₁,/2), 0.265 microns(lambda₂,/2), and 0.3 microns (lambda₃,/2), respectively. The effectivelength (Y) of bases 86, 88 from arms 90, 92 to circuit module 46 is alsoa function of the dipole antenna's resonant wavelength; for a resonantwavelength of lambda, Y is a multiple of lambda.

Referring to FIG. 5, each of the bases 86, 88 physically is made up offour segments; (1) one of the conductors 96 of link 42, (2) a portion112 of tri-bus 38, (3) a short connecting portion 124 of tri-bus 38, and(4) one of the conductors 94 of link 48, which together define a path(e.g., the path shown as dashed line 97) with an effective length of Yfrom the arm (e.g., 92) to the circuit module 46.

The placement of link 42 perpendicular to the surface of layer 34 allowsarms 90, 92 (formed on the surface of layer 34) to be spaced at anactual spacing Z that is closer than lambda/2, the minimum requiredeffective Y dimension of bases 86, 88. Spacing Z may be chosen based onthe bandwidth/gain tradeoff and for example may be one quarter of theresonant wavelength for the dipole antennas of a given section (i.e.,lambda/4, or 0.1125 microns (lambda₁/4), 0.1325 microns (lambda₂/4) and0.15 microns (lambda₃/4) for antennas 66, 68, 70, respectively).

Referring to FIG. 6, each tri-bus 38 is formed of aluminum on layer 36and has three zigzag shaped bus pairs 100, 102, 104 for respectivelyconnecting dipole antennas of the corresponding sections 60, 62, 64 oftri-dipole 30. Bus pairs 100, 102, 104 are connected to individualdipole antennas 66, 68, 70 via conductors of link 42 (FIG. 2) that arejoined to the bus pairs at points, e.g., 106.

Each bus pair 100, 102, 104 has two parallel buses 108, 110. Bus 108electrically connects together the arms of the dipole antenna 5 of thecorresponding section and, independently, the related bus 110electrically connects together the arms 92 of the dipole antennas ofthat same section.

Points 106 delineate a series of fragments 112, 114, 116 on each of thethree bus pairs 100, 102, 104, respectively. Each fragment forms part ofone or more of the bases 86, or 88 and therefore contributes to theeffective Y dimension.

The lengths (Q) of fragments 112, 114, 116 are one-half of the resonantwavelengths (i.e. lambda/2) of the sections 60, 62, 64, or 0.225 microns(lambda₁,/2), 0.265 microns (lambda₂,/2), and 0.3 microns (lambda₃,/2),respectively.

The conductors of link 48 (FIG. 2) are attached to tri-bus 38 at points118, 120, 122 at the ends of buses 108, 110. Between points 118, 120,122 and the first points 106 on along buses 108, 110 are fragments 124,126, 128, which also form portions of the bases 86, 88 and are includedto adjust the effective Y dimensions of those bases to be integermultiples of lambda/2. The lengths of the three fragments 124, 126, 128are 0.1125 microns, 0.1525 microns, and 0.1875 microns, respectively.

Referring to FIG. 7, each dipole antenna 80 is physically formed (ofaluminum) on an insulating semiconductor (e.g. silicon dioxide ofsilicon nitride) substrate 130 (part of layer 34) by x-ray or electronbeam lithography or other technique suitable for forming submicron-sizedstructures.

Tri-busses 38 (not seen in FIG. 7) are formed on the upper-side of asecond insulating semiconductor substrate 132 (part of layer 36).Circuit modules 46 (not seen in FIG. 7) are part of a third insulatingsemiconductor substrate 134 (part of layer 44) and are connected byconductors 50 to gold contact pads 136 (only one shown, not to scale)formed on the edge of substrate layer 134.

Referring to FIG. 8, circuit module 46 is formed in and on substrate 134by any one of several monolithic processes. A section 138 of thesubstrate 134, which has been previously coated with an insulatingsemiconductor oxide layer 140, is repeatedly masked (whereby smallwindows are opened in the oxide layer, exposing the semiconductorbeneath) and exposed to n and p dopants to form the desired circuitelements (not shown in detail in FIG. 8).

The individual circuit elements are connected to each other and toexternal contact pad 136 (FIG. 7) by aluminum conductors 142, 50,respectively. To form the connections, holes 144 are opened in oxidelayer 140 and a sheet of aluminum is deposited, filling holes 144. Usinga masking technique similar to the one described above the unwantedaluminum is removed, leaving only conductors 142, 50.

Semiconductor substrate layer 132 is deposited directly on top of theremaining exposed oxide layer 140 and conductors 142, 50. Holes 146 (oneshown) (opened using a suitable lithographic technique) are channels forthe electrical conductors 147 of links 48, which connect tri-bus 38 andcircuit module 46. Tri-bus 38 is etched from a sheet of aluminumdeposited onto the surface of layer 132. The deposition process fillsholes 146, thereby forming the conductors of links 48.

Substrate layer 130 is deposited onto the surface of substrate layer 132and tri-bus 38. The arms of dipole antennas 80 are formed by depositinga sheet of aluminum onto the surface of layer 130 and etching away theunwanted metal. During the deposition process holes 148 are filledthereby forming the conductors 149 of links 42 between the arms ofdipole antenna 80 and tri-bus 38.

The conductors 149 are the uppermost parts of bases 86, 88 (FIG. 4) ofdipole antennas 66, 68, 70; the lengths of conductors 149 together withthe lengths of fragments 112, 114, 116 (FIG. 6), the lengths offragments 124, 126, 128, and the lengths of the conductors 147 determinethe effective Y dimension of bases 86, 88.

The length of the conductors 149 is determined by the thickness of thesubstrate 130 through which links 42 pass. Substrate 130 and links 42are 0.05625 microns (i.e. lambda₁,/8) thick. This thickness is achievedby controlling the deposition rate of the semiconductor material aslayer 130 is formed.

The length of the conductors 149 is determined by the thickness of thesubstrate layer 132 through which they pass. This, layer and links 48are therefore also 0.05625 microns 20 thick.

The Y dimensions for the dipole antennas 66, 68, 70 of sections 60, 62,64 therefore are as follows:

(a) For section 60, Y equals the sum of 0.05625 microns (length of theconductor in link 42, lambda₁,/8)+n*0.225 microns (where 0.225microns=lambda₂/2, the length of a fragment 112, and n=the number offragments 112 in each base 86, 88 of the nth dipole antenna 66 n)+0.1125microns (length of fragment 124, lambda₄/4)+0.05625 microns (length ofthe conductor in link 48, lambda₁/8), and that sum equals(n+1)*(lambda_(1/2)).

(b) For section 62, Y equals the sum of 0.05625 microns (length of link42, lambda₁/8)+n*0.265 microns (where 0.265 microns lambda₂/2, thelength of a fragment 114, and n=the number of fragments 114 in each base86, 88 of the nth dipole antenna 68 n)+0.1125 microns (length offragment 126, (lambda₂/2)−(lambda₁/4))+0.05625 microns (length of theconductor in link 48, lambda₁/8), and that sum equals (n+1)*(lambda₁/2).

(c) For section 64, Y equals the sum of 0,05625 microns (length ofconductor in link 42, lambda₁/8)+n*0.3 microns (where 0.3microns−lambda₃/2, the length of a fragment 116, and n equals the numberof fragments 116 in each base 86, 88 of the nth dipole antenna 70n)+0.1875 microns (the length of fragment 128,(lambda₃/2)−(lambda₁/4))+0.05625 microns (length of conductor in link48), and that sum equals (n+1)*(lambda₃/2).

Referring again to FIG. 1, in some embodiments, the displayed image isnot emitted from device 20 but is comprised of ambient light (or lightfrom a source, not 25 shown) selectively reflected by the tri-dipoles 30of screen 22.

In that case, each tri-dipole 30 receives ambient light having a broadspectrum of wavelengths and is controlled by the corresponding circuitmodule to reflect only that portion of the ambient light manifesting thehue and brightness of the desired corresponding pixel.

The hue generated by tri-dipole 30 depends on the relative intensitiesof the light reflected by sections 60, 62, 64. The overall brightness ofthat hue of light in turn depends on the absolute intensities of thelight radiation reflected by sections 60, 62, 64. Thus, both the hue andbrightness of the light generated by tri-dipole 30 can be controlled byregulating the intensity of the light reflected by the dipole antennasin each section of the tri-dipole; this is done by controlling thereflectivity of each dipole antenna, i.e. the percentage of the light ofthe relevant wavelength for that dipole antenna which is reflected.

The desired percentage is attained not by regulating the amount of lightreflected at any given instant but by arranging for the antenna to befully reflective in each of a series of spaced apart time slots, andotherwise non-reflective. Each dipole antenna, in conjunction with itscircuit module, has only two possible states: either it reflects all ofthe light (at the antenna's resonant frequency), or it reflects none ofthat light. The intensity is regulated by controlling the percentage oftotal time occupied by the time slots in which the dipole antennaoccupies the first state.

Each dipole antenna is controlled to be reflective or not by controllingthe impedance of the dipole antenna relative to the impedance of themedium (e.g., air) through which the light travels. If the medium has aneffective impedance of zero, then the relationship of the reflectivityof the dipole antenna to zero (the controlled impedance of the dipoleantenna) can be derived as follows. If we define a three-axis systemx-y-z in which the x and y axes are in the plane of the array and the zaxis is the axis of propagation of the incident and reflected waves,where z=0 is the surface of the array, then the incident plus reflectedwave for z<0 may be represented as: $\begin{matrix}{\overset{\_}{\underset{\_}{E}} = {{\hat{x}{\underset{\_}{E}}_{0}{\mathbb{e}}^{- {jkz}}} + {\hat{x}{\underset{\_}{E}}_{r}{\mathbb{e}}^{+ {jkz}}}}} & (1) \\{\overset{\_}{\underset{\_}{H}} = {\frac{V \times \overset{\_}{\underset{\_}{E}}}{{- j}\quad{\omega\mu}_{0}} = {\hat{y}{\frac{1}{\eta_{0}}\lbrack {{{\underset{\_}{E}}_{0}{\mathbb{e}}^{- {jkz}}} - {{\underset{\_}{E}}_{r}{\mathbb{e}}^{+ {jkz}}}} \rbrack}}}} & (2)\end{matrix}$where E (overbar) is the complex amplitude of the electric field of thesum of the transmitted wave and the reflected wave; E ₀ is the complexamplitude of the electric field of the transmitted 20 wave; E _(r), isthe complex amplitude of the electric field of the reflective wave;x(hat) is the orientation of the electric field of the wave; H is theamplitude of the magnetic field; y(hat) is the orientation of themagnetic field; μo is the permeability of free space; ∈₀ is thepermittivity of free space; k=ωsqrt[μ₀/∈₀] is the wavenumber; and η=sqrt[μ₀/∈₀] is the impedance of free space. For z>0 (i.e., within freespace) only the transmitted wave exists and is represented by$\begin{matrix}{\overset{\_}{\underset{\_}{E}} = {\hat{x}{\underset{\_}{E}}_{t}{\mathbb{e}}^{{- {jk}_{c}}z}}} & (3) \\{\overset{\_}{\underset{\_}{H}} = {\hat{y}\frac{1}{\eta_{t}}{\underset{\_}{E}}_{t}{\mathbb{e}}^{{- {jk}_{t}}z}}} & (4)\end{matrix}$E(overbar) is the complex amplitude the transmitted wave at z=0,k_(t)=sqrt [μ∈] is its wavenumber; η=sqrt [μ/∈] is the impedance of themedium, i.e. z>0. Boundary conditions (z=0) for tangential electricfields are imposed on equations 1 and 2 and they are combined to yield,{circumflex over (x)}[E ₀ +E _(r) ]={circumflex over (x)}E _(t)  (5)In the same way, continuity for tangential magnetic fields (z=0) at theboundary yields,ŷ(1/η₀)( E ₀ −E _(R))=ŷ(1/η_(t)) E _(t)  (6)Dividing equations 5 and 6 by E ₀, and E ₀/η₀ respectively gives thefollowing two equations:1+ E _(r) /E ₀ =E _(t) /E ₀  (7)1− E _(R) /E ₀=(η₀/η_(t))( E _(t) /E ₀)  (8)E _(r)/E ₀ is called Γ and is the complex reflection coefficient whileE_(t)/η₀=T is called the complex transmission coefficient, andη_(t)/η₀=η_(n) is the normalized wave impedance. Solving for T and Γyields $\begin{matrix}{\underset{\_}{T} = {\frac{{\underset{\_}{E}}_{t}}{{\underset{\_}{E}}_{0}} = {\frac{2}{1 + {\eta_{0}/\eta_{t}}} = \frac{2\eta_{n}}{\eta_{n} + 1}}}} & (9) \\{\underset{\_}{\Gamma} = {\frac{{\underset{\_}{E}}_{R}}{{\underset{\_}{E}}_{0}} = {{\underset{\_}{T} - 1} = \frac{\eta_{N} - 1}{\eta_{n} + 1}}}} & (10)\end{matrix}$For matched impedance values, η₀=η_(n), the reflection coefficient iszero, and T=1 (i.e., no reflection), and in the case of a load at theboundary, a matched antenna, there is complete absorption.

As η_(n) approaches zero or infinity, the reflection coefficientapproaches plus or minus one, implying total reflection.

Referring to FIG. 9, the impedance z_(L), of dipole antenna 80 iscontrolled by a variable resistance PIN diode 160 connected across bases86, 88. PIN line 162 to the output of a bias high voltage or a lowvoltage based on a line 168 from power and the output of bias source 164is a high voltage, the resistance R of PIN diode 160 (and hence theimpedance (E,) of the dipole antenna is zero causing full reflection;when the output of bias source 164 is a low voltage 98, resistance R isset to a value such that the resulting impedance z@ is matched to z.(the impedance of the air surrounding the antenna), causing zeroreflection.

To generate an entire image on screen 20, power and control circuitry 26receives a video signal (e.g. a digitized standard RGB televisionsignal) and uses conventional techniques to deliver correspondingsignals to modules 46 which indicate the relative desired intensities oflight reflected from all sections 60, 62, 64 of all of the tri-dipolesin the array at a given time. Circuit modules 46 use conventionaltechniques to deliver an appropriate stream of input control signalpulses to each bias source 164 on line 168.

The pulse stream on each line 168 has a duty cycle appropriate toachieve the proper percentages of reflectance for the three Sections ofeach tri-dipole. Referring to FIGS. 10A, 10B, and 10C, for example,pulse stream 170 has a period T and a 50% duty cycle. For the first 50%of each period T the input to bias source 164 is high and theCorresponding output of source 164 is a high voltage. During thisportion of the cycle dipole antenna 80 will reflect all received lighthaving the dipole antenna's resonant wavelength. For the second 50% ofthe cycle the output of source 164 will be low and dipole antennas 80will absorb the received light. In FIGS. 10B, 10C, pulse streams 172,174 represent a 30% duty cycle and a 100% duty cycle respectively; witha 30% duty cycle the effective intensity of the light radiation of thedipole antennas of the section will be 30%; for a duty cycle of 100%,the effective intensity is 100%.

For example, if a particular pixel of the image is to be brown, therelative intensities required of the three red, 25 green, and bluesections 60, 62, 64 may be, respectively, 30, 40, and 10. The inputsignals to the bias sources 164, carried on lines 168, would then haveduty cycles, respectively, of 30%, 40%, and 10%. An adjacent pixel whichis to be a brown of the same hue but greater brightness might requireduty cycles of 45%, 60%, and 15%.

Referring to FIG. 11, to accomplish the delivery of the pulse widthmodulated signals from circuitry 26 to the pixel circuit modules 46,each circuit module 46 in the row includes storage 180, 182 for twobits. The bit 1 storage elements 180 of the modules 46 in the row areconnected together to create one long shift register with the pulsewidth modulated signals being passed along the row data line 184 frompixel to pixel. If, for example, the period of the modulated signals is1 millisecond and there are ten different intensity levels, then anentire string of bits (representing the on or off state of therespective pixels in the row during the succeeding 1/10 millisecond) isshifted down the row every 1/10 millisecond. At the end of the initial1/10 millisecond all of the bits in elements 180 are shifted to theassociated elements 182 by a strobe Pulse on strobe line 186. Thecontent of each element 182 is the input to the driver 188 for theappropriate one of the three colors of that pixel, which in turn drivesthe corresponding section 60, 62, 64 of the tri-dipole. The rate atwhich data is shifted along the shift registers is determined by thenumber of elements on a given row, the number of rows, the number ofintensity levels, and the refresh rate of the entire array.

In another embodiment, the light comprising the image is emitted bytri-dipoles 30 rather than being produced by reflected ambient light. Inthat case, each tri-dipole generates the light for a single pixel with ahue and brightness governed by the intensities of the light emitted byeach of the three sections 60, 62, 64.

Each dipole antenna within a tri-dipole is caused to emit light at theresonant wavelength of that antenna by stimulating it using a signalwhose frequency corresponds to the resonant wavelength. Thus, thesections 60, 62, 64 will emit blue (lambda₁), green (lambda₂), and red(lambda₃) light respectively when provided with signals whosefrequencies equal, respectively, lambda₁, lambda₂ and lambda₃.

For an idealized dipole, the current I and current density J(overbar)(r′overbar) are described byI=jωq  (11)J ( r ′)={circumflex over (z)}Id

( r ′)  (12)where q is the charge density; z(hat) indicates the direction of thecurrent (along the z-axis); ω is angular frequency; and d is thedistance between ideal point charges representing the dipole. The vectorpotential A(overbar) in polar coordinates is given by $\begin{matrix}\begin{matrix}{\overset{\_}{\underset{\_}{A}} = {{\hat{r}{\underset{\_}{A}}_{r}} + {\hat{\theta}{\underset{\_}{A}}_{\theta}}}} \\{= {( {{\hat{r}\cos\quad\theta} - {\hat{\theta}\sin\quad\theta}} )\frac{\mu_{0}\underset{\_}{I}d}{4\pi\quad r}{\mathbb{e}}^{- {jkr}}}}\end{matrix} & (13)\end{matrix}$where θ represents the angle relative to the dipole; θ(hat) is theangular orientation of the wave; μ₀ is the permeability of free space; ris radius from the dipole; r(hat) is radial orientation of the wave;A_(r) is the radial component of the vector potential; A_(θ) is theangular component of the vector potential; and k is a factor which isused to represent sinusoidally varying waves. The H field is given bywhere φ is elevation, with respect to the dipole. The E field is givenby,The far-field equation is given by $\begin{matrix}\begin{matrix}{\overset{\_}{\underset{\_}{H}} = {\varphi{\frac{1}{\mu_{0}r}\lbrack {\frac{\delta}{\delta\theta}( {{r{\underset{\_}{A}}_{\theta}} - {\frac{\delta}{\delta\theta}( {\underset{\_}{A}}_{r} )}} \rbrack} }}} \\{= {\varphi\frac{{jk}\underset{\_}{I}d}{4\pi\quad r}{{\mathbb{e}}^{- {jkr}}\lbrack {1 + \frac{1}{jkr}} \rbrack}\sin\quad\theta}}\end{matrix} & (14) \\\begin{matrix}{\overset{\_}{\underset{\_}{E}} = {\frac{1}{j\quad\omega\quad\varepsilon_{0}}{\nabla{\times \overset{\_}{\underset{\_}{H}}}}}} \\{= {\sqrt{\mu_{0}/\varepsilon_{0}}\frac{{jk}\underset{\_}{I}d}{4\pi\quad r}{{\mathbb{e}}^{- {jkr}}( {{{\hat{r}\lbrack {\frac{1}{jkr} + ( \frac{1}{jkr} )^{2}} \rbrack}2\cos\quad\theta} +} }}} \\ {{\theta\lbrack {1 + \frac{1}{jkr} + ( \frac{1}{jkr} )^{2}} \rbrack}\sin\quad\theta} )\end{matrix} & (15) \\{{\underset{\_}{\overset{\_}{H}} = {\hat{\varphi}\frac{{jk}\underset{\_}{I}d}{4\pi\quad r}{\mathbb{e}}^{- {jkr}}\sin\quad\theta}}{\underset{\_}{\overset{\_}{E}} = {\hat{\theta}\sqrt{\mu_{0}/\varepsilon_{0}}\frac{{jk}\underset{\_}{I}d}{4\pi\quad r}{\mathbb{e}}^{- {jkr}}\sin\quad\theta}}} & (16)\end{matrix}$

Equation (16) describes the radiation pattern away from a dipole antennaat distances significantly greater than the wavelength of the emittedelectromagnetic wave. It is a very broad radiation pattern providing awide field of view at relevant distances.

Referring to FIG. 12, the dipole antennas 66, 68, 70 of each section 60,62, 64 are driven by signals (e.g., sinusoidal) with frequencies of5×10¹⁴ Hz, 5.6×10¹⁴ Hz, and 6.6×10¹⁴ Hz for red, green, and blue,respectively. These signals are supplied by three monolithic oscillators200 (one shown) within circuit module 46, each tuned to one of the threerequired frequencies.

In circuit 200 (an a stable multivibrator), the center pair of coupledtransistors 202, 204 are the primary active elements and will oscillateif the circuit admittance's are set appropriately. Diodes 206, 208, 210,212 provide coupling capacitance's between the transistors and theinductors 214, 216 are used to tune the operating frequency.

In a third embodiment, an image of the object is focused by aconventional lens (not shown in FIG. 1) onto screen 22, which then actsas an image sensor. The tri-dipoles of screen 22, controlled by powerand control circuitry 26, generate electrical signals corresponding topixels of the received image. The signals are then processed by aprocessor which, in conventional fashion, delivers a derived RGB videosignal which can then be transmitted or stored.

The signals generated for each tri-dipole are generated by thecorresponding circuit module 46 and represent the hue and brightness ofthe light radiation received at that tri-dipole.

Each section of tri-dipole 30 can only be used to measure light havingthe resonant wavelength of its respective dipole antennas, however,because most colors can be expressed as a combination of red, green, andblue, circuit module 46 can, by independently measuring the intensity ofthe light radiation received at each section 60, 62, 64, derive a signalwhich specifies the hue and intensity of the received pixel.

Referring to FIG. 13, dipole antenna 80 will absorb incident lightradiation at its resonant wavelength when its reflection coefficient(Γ_(L)) is zero, which occurs when its controlled impedance (z_(L))matches the impedance of the medium (z₀). In those circumstances, avoltage pulse is produced across the ends 308, 310 of dipole 80 for eachincident photon. The relative magnitude of the light radiation receivedby each dipole antenna can thus be measured by counting the averagenumber of pulses across ends 308, 310 over a given time period.

In this embodiment, circuit module 46 includes a terminating loadresistor 315 connected across ends 308, 310. The controlled impedance ofthe combination of dipole antenna 80 and resistor 315, described by theequations set forth below, is equal to z₀.

The voltage of the pulse across resistor 315 (created by an incidentphoton) is illustrated by the sine wave graph above register 15 and isdescribed generally by the following equationV(z)= Ve ^(−jkz)+Γ_(L) e ^(jkz)  (17)Because z_(L)=z₀, Γ_(L)=0, and equation 17 simplifies toV(z)= V+e ^(−jkz)  (18)

A pulse detector 318 amplifies and sharpens the resulting pulse to asquare wave form as shown, which is then used as the clock (CLK) input319 to a binary counter 320. The output of the binary counter is sampledat a regular rate; collectively the samples form a digital signalrepresenting the intensity of received light radiation over time. Eachtime counter 320 is sampled, it is reset to zero by a pulse on controlline 322, Counter 320 thus serves as a digital integrator that indicateshow much light arrived in each one of a succession of equal length timeperiods.

Referring to FIG. 14, in pulse detector 318 the pair of transistors 322,324 serve as a high impedance differential stage whose output(representing the voltage difference between points 308, 310) isdelivered to an amplifier 326. Amplifier 326 serves as a high-bandwidthgain stage and delivers a single sided output pulse to a conditioningcircuit 328 that converts slow rising pulses to square pulses 330 fordriving counter 320.

In another embodiment, the array of tri-dipoles is operated as a phasedarray. The operation of phased arrays is discussed more fully in Amitay,et al., Theory and Analysis of Phased Array Antennas, 1972, incorporatedherein by reference. By controlling the spacing of successivetri-dipoles across the array and the relative phases of their operation,wave cancellation or reinforcement can be used to control the directionin three dimensions and orientation of the radiation. Beams can thus begenerated or scanned. In the case of an array used to sense incomingradiation, the array can be made more sensitive to radiation receivedfrom selected directions.

Other embodiments are also possible. For example, referring to FIG. 15,each section of tri-dipole 400 array be a single dipole antenna 406,407, 408. The tri-dipole antennas are then arranged about a center Point410 at 120 degree intervals in a radial pattern. Bases 411, 412, as wellas arms 414, 415, of the dipole antennas, are all formed on the samesurface.

Referring to FIG. 16, each section may consist of multiple dipoleantennas 406, 407, 408 connected by attaching the bases 411, 412 of eachsucceeding dipole antenna to the inner ends of arms 414, 415 of thepreceding dipole antenna. Circuit modules 416 are formed on the surfaceof layer 413.

Referring to FIG. 17, a multi-dipole 430 could have five sections 432,434, 436, 438, 440 composed of dipole antennas 442, 444, 446, 448, 450,respectively. The dipole antennas of the different sections would havedifferent resonant wavelengths. Other multi-dipoles might have anynumber of sections.

The scanning of pixels could be done other than by pulse widthmodulation, for example, using charge coupled devices to shift packetsof charge along the rows of pixels.

Referring to FIGS. 18, 19, other arrangements of dipole antennas may beused in order to match the area required for the control circuitmodules.

Referring to FIG. 18, each section 470 of a tri-dipole in the reflectivemode could be formed of a number of subsections (e.g., 472) arranged intwo rows 474 and a number of columns 47. The antennas 478 in eachsubsection 472 are all served by a single PIN diode circuit 480 locatedat the peripheral edge of section 470 at the end of the subsection onthe layer below the antenna layer. All circuits 480 for the entireSection 470 are in turn served by a single bias source 164 (FIG. 9).This arrangement reduces the number of bias sources required for theentire array of tri-dipoles. FIG. 19 shows an alternate arrangement inwhich there is but one row of subsections each served by a single PINdiode circuit at the end of the row.

In order to reduce the number of conductors 50, selected tri-dipolescould be used to receive control signals transmitted directly by lightand to pass those control signals to the control circuits of nearbyactive tri-dipoles.

The dipoles could be mono-dipoles comprised of only a single dipoleantenna, all with the same resonant wavelength.

Dipole antennas 470 could be randomly arranged on the surface of layer472 of screen 22.

A different color regime, e.g. cyan-magenta-yellow, could be substitutedfor RGB.

Spiral, biconical, slotted, and other antenna configurations could besubstituted for the dipole configuration.

The array could be three-dimensional.

The successive tri-dipoles in the array can be oriented so that theirrespective antennas are orthogonal to each other to enable the array tointeract with radiation of any arbitrary polarization.

The PIN diodes could be replaced by other impedance controllingelements. Such elements might include quantum well transistors,superconducting junctions, or transistors based on vacuummicroelectronics. Further improvement could be achieved by reducing thecomplexity of the third layer containing control circuitry. Theelectronics required to get control signals to the circuitry could beeliminated by the use of laser or electron beams to provide suchsignals. This would have the advantage of allowing for arrays of evenhigher density.

The array could be fabricated on a transparent substrate, thusfacilitating transmissive operation.

In other embodiments, the antenna arrays alone (without controlcircuitry or connection buses) may be fabricated on one-half of amicrofabricated interferometric cavity. The antenna array can beconsidered a frequency selective mirror whose spectral characteristicsare controlled by the dimensions of the antennas. Such a cavity willtransmit and reflect certain portions of incident electromagneticradiation depending on (a) the dimensions of the cavity itself and (b)the frequency response of the mirrors. The behavior of interferometriccavities and dielectric mirrors is discussed more fully in Macleod, H.A., Thin Film Optical Filters, 1969, incorporated by reference.

Referring to FIG. 20 a, two example adjacent elements of a larger arrayof this kind include two cavities 498, 499 fabricated on a transparentsubstrate 500. A layer 502, the primary mirror/conductor, is comprisedof a transparent conductive coating upon which a dielectric or metallicmirror has been fabricated. Insulating supports 504 hold up a secondtransparent conducting membrane 506. Each array element has an antennaarray 508 formed on the membrane 506. The two structures, 506 and 508,together comprise the secondary mirror/conductor. Conversely, theantenna array may be fabricated as part of the primary mirror/conductor.Secondary mirror/conductor 506/508 forms a flexible membrane, fabricatedsuch that it is under tensile stress and thus parallel to the substrate,in the undriven state.

Because layers 506 and 502 are parallel, radiation which enters any ofthe cavities from above or below the array can undergo multiplereflections within the cavity, resulting in optical interference.Depending on the dimensions of the antenna array, as explained above,the interference will determine its effective impedance, and thus itsreflective and/or transmissive characteristics. Changing one of thedimensions, in this case the cavity height (i.e., the spacing betweenthe inner walls of layers 502, 506), will alter the opticalcharacteristics. The change in height is achieved by applying a voltageacross the two layers at the cavity, which, due to electrostatic forces,causes layer 506 to collapse. Cavity 498 is shown collapsed (7 voltsapplied), while cavity 499 is shown uncollapsed (0 volts applied).

In another embodiment, FIG. 20 b, each cavity may be formed by acombination of dielectric or metallic mirrors on the two layers, andwithout the antennas formed on either layer. In this case the spectralcharacteristics of the mirror are determined by the nature andthickness(es) of the materials comprising it.

In an alternative fabrication scheme, FIG. 20 c, each cavity isfabricated using a simpler process which precludes the need forseparately defined support pillars. Here, each secondarymirror/conductor, 506, is formed in a U-shape with the legs attached tothe primary layer; each secondary mirror/conductor thus isself-supporting.

In yet another scheme, FIG. 20 d, the cavity has been modified to alterit's mechanical behavior. In this version, a stiffening layer, 510, hasbeen added to limit deformation of the membrane while in the drivenstate. This assures that the two mirrors will remain parallel as adriving voltage is gradually increased. The resulting device can bedriven in analog mode (e.g., cavity 511 may be driven by 5 volts toachieve partial deformation of the cavity) so that continuous variationof its spectral characteristics may be achieved.

Referring to FIGS. 21A and 21B, the modulation effect on incidentradiation is shown. The binary modulation mode is shown in FIG. 21A. Inthe undriven state (shown on the left) incident light 512 (the deltalambda represents a range of incident frequencies, e.g., the range ofvisible light) contains a spectral component which is at the resonantfrequency of the device in the undriven state. Consequently thiscomponent (delta lamba n) is transmitted, 516, and the remainingcomponents (at nonresonant frequencies, delta lambda minus delta lamban) are reflected, 514. This operation is in the nature of the operationof a fabry-perot interference cavity.

When the device is driven and the geometry altered to collapse (rightside of figure), the resonant frequency of the device also changes. Withthe correct cavity dimensions, all of the incident light (delta lamba)is reflected.

FIG. 21A shows a binary mode of operation while FIG. 21B shows an analogmode, where a continuously variable voltage may be used to cause acontinuously variable degree of translation of secondarymirror/conductor 506. This provides a mechanism for continuous frequencyselection within an operational range because the resonant frequency ofthe cavity can be varied continuously. In the left side of FIG. 21A, thetransmitted wavelengths are delta lambda n zero, while in the right handside they are delta lambda n one.

The equations which explain the performance of the cavity are set forthat the bottom of FIG. 21B. Equation 1 defines the transmission T througha fabry-perot cavity. Ta, Tb, Ra, Rb are the transmittances andreflectances of the primary (a) and secondary (b) mirrors. Phi a and Phib are the phase changes upon reflectance at the primary and secondarymirrors, respectively. Delta is the phase thickness. Equation 2 definesthe phase thickness in terms of the cavity spacing ds, the index ofrefraction of the spacer ns, and the angle of incidence, theta s.Equation 3 shows that the transmission T becomes the transmission of thesecond mirror when the transmission of the first mirror approaches 0.

There are a number of particular frequency response modes which would beuseful in applying the invention to displays. FIGS. 22A through 22Fillustrate some of the possibilities and relate to the equations of FIG.21B. These are idealized plots which illustrate transmission andreflectivity (T/R) of the cavity for wavelengths in the visible range indriven and undriven states for each of the driven and undriven responsemodes. The different modes are achieved by using different combinationsof mirrors and cavity spacings.

The spectral characteristics of the mirrors used can be referred to asbroad-band and narrow-band. The mirrors are optimized for the visiblerange with a broad band mirror operating across the entire visible range(i.e., reflecting over a minimum range of 400 nm to 700 nm). Such amirror is denoted in the stack formula 1.671

0.775(ERS) 0.833M (ERS)

11.671 where ERS denotes an equal ripple filter. The ERS filter haslayers which are a quarter wavelength thick, and their refractiveindices are n0=1.671, n1=1.986, n2=1.663, n3=2.122, n4=1.562, n5=2.240,n6=1.495, n7=2.316, n8=1.460, n9=2.350, n10=1.450. A narrow-band filteroptimized for the color green would reflect only over the range of 500nm to 570 nm, and transmit everywhere else. Such a filter is describedby the stack formula 1

C1 2C2 (3A/2 3B1 3A/2)2 (3A/2 3B 3A/2)6 (3A/2 3B1 3A/2)2

1.52 where the refractive indices are nA=0.156, nC2=nB1=1.93, andnB=2.34.

The cavity spacing (i.e., cavity height) in both driven and undrivenstates can be set to a predetermined value by the film thicknesses usedin its construction. These two values determine whether a cavity isresonant or non-resonant. For a resonant cavity, the spacing isdetermined such that the undriven state coincides with the resonant peakof the narrower of the two mirrors. When a device is non-resonant, itmust be driven in order for the device to become resonant.

For example, if the undriven cavity spacing were 535 nm then, becausethis coincides with the center of the previously defined narrow-bandmirror, there would be a transmission peak at this frequency. Peakspacing for this cavity is 267 nm so the other peaks, which would occurin the standard Fabry-Perot fall outside of range of the narrow bandmirror. This would be considered a resonant cavity because the peakoccurs in the undriven state. Driving the cavity so that the spacingwere 480 nm would result in no transmission peak because all of thecavity peaks are outside the range of the narrow-band mirror. For allpractical purposes the narrow-band mirror does not exist at thisfrequency and therefore the transmission peak disappears.

FIG. 22A shows a T/R plot of a cavity having broad band mirrors on bothlayers of the cavity. When undriven, this results intransmission/reflection peaks which occur at wavelengths which areapproximately integral multiples of the cavity spacing. (the notation mdelta lambda n in FIG. 21A denotes the fact that there may be a seriesof peaks.) This is classic fabry-perot behavior. In the driven state(shown to the right in FIG. 22A), the cavity resonance is shifted out ofthe visible range causing the device to act like a broadband mirror.

FIG. 22B shows a T/R plot for a cavity having one broad band and onenarrow band mirror. This device has a resonant cavity, causing atransmission peak at the center of the narrow-band mirror's passbandwhen the device is in the undriven state. Driving the device (right handside of FIG. 22B) shifts the cavity resonance away from that of thenarrow band mirror, and the device acts like a broadband mirror.

In FIG. 22C, the cavity is like that of FIG. 22B, except the cavity isnon-resonant which results in broadband mirror cavity behavior in theundriven state. When driven, the cavity spacing shifts into resonance,causing a transmission peak centered on the narrow-band mirror.

FIG. 22D shows the performance of a resonant cavity with two narrow-bandmirrors. When undriven, there is a transmission peak centered on themirrors' response. Since the mirrors are narrow-band, the overall cavityresponse is that of a broad-band transmitter. Driving the device out ofresonance (i.e. active) results in a reflective peak at the narrow-bandcenter frequency.

Like FIG. 22D, the cavity of FIG. 22E has two narrow band mirrors, butit is a non-resonant cavity. Consequently its behavior is opposite thatof the cavity portrayed in FIG. 22D.

Thus, when one of the mirrors is narrow banded, mirror a for example,the transmission approaches zero for frequencies outside its range. Thisis essentially the transmission of mirror b. When both of the mirrorsare narrow banded, the transmission becomes a maximum outside thefrequency range. In either case, the spurious peaks typical of afabry-perot are avoided. The result is a device which can be describedas a single node resonant cavity.

When both of the mirrors are narrow banded, then fabry-perot typebehavior occurs only when the cavity spacing is correct. Making themirrors narrow enough allows only a single peak to occur. Then it isunnecessary to be concerned about spurious peaks that might occur withinthe visible range.

FIG. 22F is for a cavity with a simpler design involving only a metallicmirror on one wall and a hybrid filter on the other wall. The hybridfilter is a combination of a narrow bandpass filter (outer surface) andan induced absorber (inner surface). The induced absorber is a simplestructure which can consist of one or more dielectric or dielectric andmetallic films. The function of the absorber is to cause incident lightof a specified frequency range to be absorbed by a reflective surface(i.e. mirror). One such design can be achieved with a single film ofrefractive index n=1.65 and a thickness of 75.8 nm. The induced absorberonly functions when it is in contact with the mirror, otherwise it isinconsequential.

In the undriven state, the hybrid filter (a green centered narrowbandpass/induced absorber) reflects everything but the green light,which is unaffected by the induced absorber and subsequently reflectedby the metallic mirror. Thus the overall cavity response is like that ofa broad-band mirror. When in the driven state, the hybrid filter comesinto contact with the metallic mirror. The absorber couples the greenlight into the mirror, and the result is an absorption peak at thatwavelength.

Referring to FIG. 23A, a red 3×3 pixel (i.e., 9 cavities) subtractivemode display array based on the cavity device using the N-N (narrowband-narrow band) configuration of FIG. 22D is shown. The cavity pixelsare formed at the intersections of primary mirror/conductors 602 andsecondary mirror/conductors 604. The display is fabricated on substrate608 and driven via contact pads 606 connected to each conductor/mirror604.

A full nine-pixel display comprises three replications of the array ofFIG. 23A arranged on top of one another and fabricated on separatesubstrates or color planes 610, 612, 614, as shown in FIG. 23B. Each ofthe individual color planes interacts only with and reflects one color(e.g., red, green, or blue), while passing all other colors. This isdone by selecting the mirror spectral characteristic and cavity spacingin each color plane appropriately. The color planes are physicallyaligned and electrically driven through the contact pads to produce fullcolor images. The image would be viewed from below in FIG. 23B.

Referring to FIGS. 23C and 23D, a single layer composite approach isshown. Such a device would be more complicated to fabricate (though themirror designs are simpler) but may suffer from inferior resolution. Inthis case, all three colors reside on the same array 616. Devices usingeither the B-B, B-N, N-N, or the M-F (B=broad band, N=narrow band,M=mirror, H-F=hybrid filter) configuration are used for this display.

Either the three plane or the single plane approach may be used ineither transmissive and reflective modes. Pixel size and overall displaysize can be altered to make the displays useful in many differentdisplay and spatial light modulator applications. These include directview and projection displays, optical computing, holographic memory, andany other situation where a one or two dimensional modulator of lightcan be used.

Because these structures depend on electrostatic attraction, whichvaries in an exponential fashion with cavity spacing, while themechanical restoring force of the membrane varies linearly with cavityspacing, they exhibit hysteresis. As seen in FIG. 24A, the straight linelabelled membrane tension shows that the restoring force on the membranevaries inversely linearly with distance (i.e., cavity spacing). That is,the smaller the spacing, the stronger the mechanical force tending torestore the original, at rest spacing. On the other hand, electricalattraction between the two layers of the cavity (the curved line) variesexponentially with smaller spacing, that is, as the two layers getcloser there is an exponential increase in the attractive force betweenthem. This causes a hysteresis effect as follows. With a low drivingvoltage applied, the secondary mirror/conductor experiences adisplacement towards the substrate until the force of restorationbalances the electrical attraction. However if the voltage is raisedpast a point known as a collapse threshold, the force of restoration iscompletely overcome and the membrane is pressed tightly against thesubstrate. The voltage can then be lowered again to some degree withoutaffecting the position of the membrane. Only if the voltage is thenlowered significantly or eliminated will the membrane be released.Because the membrane is under tensile stress, it will then pull itselfaway from the substrate when the voltage is released. This hysteresiscan be used to achieve matrix addressing of a two-dimensional array, asexplained with reference to FIG. 24B.

The display can be addressed and brightness controlled using controlpulse sequences in the driving voltage. Shown is a timing diagram for a3×3 pixel array analogous to that shown in FIG. 23A. During operation, acontinuous series of −5 volts scanning pulses is applied to the rows(rows 1-3) of the pixel array in a sequential fashion, as seen in thecharts labelled “Row”. The pulses appear at the same frequency on eachof the rows but the pulses for different rows are staggered. Thesepulses are insufficient in their own right to cause the membrane tocollapse. The columns (cols. 1-3) of the pixel array (see chartslabelled “Col) are maintained at a bias voltage of 5 volts so that thenominal voltage across each unactivated pixel is 5 volts. At times whenthe scan pulses are delivered to that pixel, the nominal row and columnpotentials are 5 and −5 volts respectively, resulting in a cavityvoltage of 10 volts. With a 10 volts potential applied to the row and a−5 volts potential to the column, the total voltage across the cavitybecomes 15 volts which is sufficient to drive the secondarymirror/conductor into the collapsed state, where it will remain untilthe end of the scan when all of the column voltages are pulsed to zero.The three charts at the bottom of FIG. 24B show the on and off states ofthe three pixels identified there by row and column numbers.

The intensity or brightness of a pixel may be varied by changing thefraction of the scan during which the pixel is activated. The scan cyclebegins at 198 and ends at 199. The frequency of the scan pulses is suchthat six pulses of a given row fall within the scan cycle, providing anopportunity to activate a pixel at any one of six times during eachcycle. Once the pixel is activated it stays on until the end of thecycle. Therefore six different intensities are possible for each pixel.For the scan shown, pixel C1R1 is at full brightness, pixel C2R2 is at4/6 brightness, and pixel C3R2 is at ⅙ brightness. All pixels arecleared at the end of the scan and the cycle begins again. Since thesestructures can be driven at frequencies as high as 50 kHz, this methodof brightness control is practical. Assuming six brightness levels,there would be a possibility of more than 8 thousand row scans persecond.

Two processes for fabricating the arrays will be discussed; others mayalso be possible.

Referring to FIG. 25A, substrate 700 is first cleaned using standardprocedures. The substrate may be of many different materials includingsilicon, plastic, mylar, or quartz. The primary requirement is that thematerial be able to support an optically smooth, though not necessarilyflat, finish. A preferred material would likely be glass, which wouldpermit both transmissive and reflective operation in the visible range.

The substrate is then coated with the primary conductor/mirror layer(s)702. This can be achieved using a physical vapor deposition (PVD) methodsuch as sputtering or e-beam evaporation. Other possible methods includechemical vapor deposition and molecular beam epitaxy. The dimensions andnature of the layer(s) depend on the specific configuration desired.Detailed examples are discussed below.

Referring to FIG. 25B, a photoresist 704 has been patterned on theprimary conductor/mirror. The photoresist may be of a positive ornegative type. The standard procedure for this step involves spinning ofthe resist, softbaking at 90 C, exposing through an appropriate mask,developing to produce the pattern, and hardbaking at 130 C.

Referring to FIG. 25C, the photoresist pattern is defined in the primaryconductor/mirror by an etching process. This step can be achieved eitherby wet chemical means or by plasma or reactive ion etching (RIE). Thechoice of etching technique depends on the nature of theconductor/mirror. In the case of an aluminum conductor/mirror, chlorinegas may be used to perform the etch, with a standard chamber power of100 watts producing an etch rate of 100 angstroms/min. Some mirrormaterials may resist RIE and in such cases a technique such as ionmilling may be used. All RIE steps are performed at a pressure of 30mtorr unless otherwise noted. All plasma etch steps are performed at apressure of 100 mtorr unless otherwise noted. The photoresist is removedusing standard solvents.

Alternatively, the conductor/mirror may be defined using the well-knownprocedure called lift-off. This procedure is used to define a layer in asubsequent step and is described below.

Referring to FIG. 25B, support rail material 706, has been depositedusing one of the methods mentioned previously. This material should bean insulator, for example silicon dioxide or silicon nitride. Thematerial should be deposited uniformly, and at a thickness equal tothickness of the spacer layer, which will be deposited later. Thisthickness should in general be at least a multiple of the wavelength oflight of interest. A thickness of 0.5 microns would place such a devicenear the middle of the visible spectrum.

Referring to FIG. 25E, photoresist layer 708 is spun on and hardbaked.Since this layer will not be photolithographically defined, otherpolymeric materials may be used instead. The only requirement is thatthey dissolve in solvents such as acetone or methanol, and be able towithstand a vacuum. This is the first step in defining a lift-offstencil.

Referring to FIG. 25F, template layer 710 has been deposited using oneof the methods of PVD. The layer is of silicon dioxide though othermaterials are possible. Ideally the material should be etched using aprocess which does not affect the underlying resist. Buffered Oxide Etch(BOE) which consists of Hydrofluoric acid diluted 7:1 with water canperform this step in 15 seconds. The layer need only be a thousandangstroms thick.

In FIG. 25G, photoresist layer 712 has been spun-on and patterned in amanner already discussed.

Referring to FIG. 25H, using a combination of BOE and RIE, the patternof resist layer 711 has been transferred to layers 710 and 708. In thefirst step, the BOE is used to etch through the silicon dioxide layer710. An oxygen plasma is used to etch through resist layer 708, and toremove resist layer 711. Plasma etching differs from RIE in that ittends to be less anisotropic, yielding profiles that are not purelyvertical.

Referring to FIG. 25I, using an oxygen plasma, resist layer 708 has beenslightly underetched in order to facilitate removal of the lift-offstencil. RIE using a carbon tetrafluoride chemistry (CF4/O2 6:4) is thenapplied to etching through layer 706.

Referring to FIG. 25J, spacer layer 712 is deposited using PVDtechniques. This material can be any number of different compounds whichcan be deposited using this technique. There are two key requirementsfor such a material. The first is that the material be soluble in waterbut not in solvents such as acetone or methanol. Example materialsinclude lithium fluoride, aluminum fluoride, and sodium chloride. Thesecond is that it be deposited with extreme uniformity and thicknesscontrol.

The former allows resulting structures to be underetched without damageby using water as the final etchant. Water is an extremely benignsolvent, and makes possible the incorporation of many different mirror,conductor, and structural materials in the final device.

The latter insures that subsequent layers conform to the variations ofthe substrate and therefore primary conductor/mirror. This parallelismis essential for the optical behavior of the resonant cavity devices.

The spacer may also be composed of a polymeric material such ashardbaked photoresist or polyimide. To achieve the required thicknessuniformity, a technique other than spinning must be used to deposit thepolymer. Two such techniques include extrusion and capillary coating.The consequence of using such a spacer is that all subsequent processsteps must be low temperature in nature to prevent outgassing andshrinkage of this layer. In this case, the spacer is ultimately removedusing an oxygen plasma.

The stencil is subsequently removed using an ultrasonic acetone bath andmethanol rinse. This also removes or lifts off excess deposited spacermaterial and is what constitutes the final step of the lift-off process.

Alternatively, by using negative photoresist and an oppositely polarizedmask, a natural overhang may be produced via overexposure. The same maybe accomplished with positive photoresist using a technique known asimage-reversal. This would preclude the need to put down a sacrificialphotoresist layer and a subsequent SiO2 layer.

Referring to FIG. 25K, secondary conductor/mirror layer(s) and supportmembrane (714) are deposited. The nature of the conductor/mirror isdependent on the application. The support membrane must have a tensileresidual stress. Tensile stress is required for two reasons. First sothat the resulting membranes will be flat and therefore parallel to thesubstrate in the quiescent state. Secondly, such structures have atendency to stick when the membranes come in contact with the substrate.Sufficient tensile stress is required pull the membrane away when thedrive voltage is reduced.

The membrane must have the appropriate optical characteristics as well.For visible light this would mean transparency in the visible region.Silicon nitride is one candidate for this role for it can be depositedusing plasma enhanced chemical vapor deposition (PECVD) with controlledstress. Other candidates include silicon dioxide, magnesium fluoride andcalcium fluoride, all of which can be deposited using e-beam evaporationwith a resulting tensile stress.

In FIG. 25L, photoresist layer 716 has been spun-on and patterned in amanner discussed above.

In FIG. 25M, using RIE or ion milling, layer(s) 714 have been etchedaccording to the pattern of resist layer 716.

Referring to FIG. 25N, the final etch is accomplished by placing thedevice in water for a period of time. The water is agitated, and whenthe devices are fully etched they are dried.

One variation on this step involves the use of n-butyl alcohol todisplace the water when the etch is finished. The devices are thenplaced in a chamber at approximately 32 degrees centigrade to cause thealcohol to freeze. After this step the devices are placed into a vacuumchamber where the air is then evacuated. This causes the alcohol tosublime and can reduce membrane sticking during the drying phase.

Another alternative process has initial steps of assembly shown in FIGS.26A through 26C, analogous to those shown in FIGS. 25A through 25C.

Thereafter, in FIG. 26D, photoresist or polymer layer 806 is spun on anda stencil layer, 208, of silicon dioxide is deposited using PVD. Thislayer must be thicker than the spacer layer to be deposited.

In FIG. 26E, resist layer 810 has been spun-on and patterned usingstandard procedures.

In FIG. 26F, this step uses BOE and an oxygen plasma etch to define alift-off stencil.

In FIG. 26G, the spacer material is chosen and deposited as described inFIG. 25J.

In FIG. 26H, the stencil is subsequently removed using an ultrasonicacetone bath and methanol rinse.

The step shown in FIG. 26I is analogous to that shown in FIG. 25K.

In FIG. 26J, photoresist layer 814 has been spun-on and patterned.

In FIG. 26K, using RIE or ion milling, layer(s) 812 have been etchedaccording to the pattern of resist layer 214.

The final etch is accomplished in a manner described above.

All of the materials used for the mirrors must be deposited in such away that their stress can be controlled. Ideally, they are deposited as“stress balanced” which means that the overall stress of the film orfilm stack is zero. Since the ultimate goal is that the support membraneconductor/mirror combination have an overall tensile stress,conductor/mirror films with compressive stress may be accommodated byhaving a high stress support membrane. The technique of ion assistede-beam deposition (IABD) precludes the need for such accommodation.Using this technique, the residual stress of almost any film of interestmay be controlled by bombarding the substrate with a stream of neutralions during the deposition process. This make possible the totalmechanical decoupling of the support material from the optical material.As a result, emphasis can be placed on depositing an optically neutralsupport membrane with ideal mechanical characteristics. In the samemanner, a “mechanically neutral” (i.e. stressless) conductor/mirror canbe deposited with ideal optical characteristic.

Referring to FIG. 27A, the simplest conductor/mirror configuration foran individual cavity is formed from a layer 900 that is either thesubstrate for the primary conductor/mirror, or the support membrane ifthis is the secondary. Layer 902 is a metallic film with a thickness onthe order of several hundred angstroms. The film can be of aluminum,silver, or any number of metals, based on the spectral, and resistiveproperties as well as the ease with which the metal can be etched.

The use of a metal as both mirror and conductor simplifies fabrication.Unfortunately the spectral characteristics of the metallic layer cannotbe tailored, and the performance limits devices to very specific kindsof behavior. Layer 904 is an insulator and/or reflection enhancementfilm. This can be formed by oxidizing the metal, if aluminum is beingused, in an oxygen plasma thus forming a thin layer of aluminum oxide.Alternatively, insulating layers or reflection enhancement layers may bedeposited in a manner discussed before. Metallic mirrors must besomewhat transmissive and therefore no more than several hundredangstroms thick. Insulator films can have thicknesses from one hundredto several thousand angstroms. Their thickness is determined by the kindof voltages expected in driving the devices.

Referring to FIG. 27B, a more elaborate configuration is shown. This isa compound conductor/mirror, with layer 900 as the substrate or thesupport membrane. The conductor 906 is either a transparent film such asindium tin oxide (ITO), or a very thin metallic layer such as gold.Either can be deposited using suitable film deposition methods.Thicknesses for the ITO should be in the range of several thousandangstroms, and metallic conductors less than 100 angstroms. 908 is amultilayer dielectric stack comprising the mirror. Such a mirrorconsists of alternating dielectric films with differing indexes ofrefraction deposited by a suitable PVD process. By choosing films withappropriate thicknesses and indexes, mirrors with tailorable spectralcharacteristics can be fabricated as is well known in the art. Ingeneral, the thickness of the individual layers is one quarter thewavelength of the light of interest.

Alternatively, these mirrors may be deposited using a technique known ascodeposition. In this case, PVD is used to deposit two materials withdifferent refractive indices simultaneously. Using computer control therefractive index of the resulting film can be varied continuouslybetween those of either film. This deposition technique makes possiblemirrors with virtually any spectral characteristic.

The ability to design the characteristics of this mirror allow fordevices with a greater variety of modes of operation. Unfortunately,because the conductive layer is not perfectly transparent, additionallosses are incurred as a result.

Referring to FIGS. 27C and 27D, a dielectric mirror 908 is depositeddirectly on substrate 900. Metallic conductor 902 and insulator 904 aredeposited and patterned such that they form a border around theperiphery of the mirror. Using this configuration, it is possible toprovide drive voltages to the devices without compromising throughputsince the conductor can be placed outside the active area of the device.FIG. 27D shows a planar view of this mirror configuration.

Response times of this device may suffer as a result of decreasedconductor area.

Referring to FIG. 28 a, a linear tunable filter is shown which has beenfabricated using the process sequence defined above. The majordifference is the nature of the mask used to define the self-supportingmembrane, which is comprised of support 1006 and 1008. The substrate,1000, is transparent in the frequency region of interest, and electrodes1004 are used to drive the device. Dielectric mirror 1002 are definedseparately to produce a configuration like that of FIGS. 27 c, 27 d.Three filters are shown though many more can be fabricated. Each filter1010, 1012, and 1014 is driven independently so that individualfrequencies may be separated from an incident light beam. Such a devicecan find use in spectroscopic analysis, as a demultiplexor in awavelength division multiplexed fiber optic communication system, acolor printer, or any other application where independent frequencyselection is required. FIG. 28 b is a top view of the structure.

Referring to FIGS. 29 a and 29 b, a device known as a deformable mirrorincludes a self-supporting membrane 1102 fabricated on a substrate 1100.When a potential is applied between actuator electrodes 1104 andconducting mirror 1106, the surface of the mirror can be deformed in acontrollable manner. Such a device can be used as a component in anadaptive optics system, or in any situation where control of an incidentwavefront is required.

Other embodiments are within the scope of the following claims.

1. A device for modulating light in the visible spectrum comprising anarray of modulation elements, and control circuitry connected to thearray for controlling each of the modulation elements independently,each of said modulation elements having a surface which is caused toexhibit a predetermined impedance characteristic to particularfrequencies of light.
 2. The device of claim 1 wherein the surfacecomprises antennas configured to interact with selected frequencies oflight.
 3. The device of claim 1 wherein the surface comprises a surfaceof an interference cavity.
 4. The device of claim 1 wherein theimpedance characteristic comprises reflection of particular frequenciesof light.
 5. The device of claim 1 wherein the impedance characteristiccomprises transmission of particular frequencies of light.
 6. The deviceof claim 1 wherein each of the modulation elements comprises aninterference cavity that is deformable to alter the cavity dimension. 7.The device of claim 6 wherein the interference cavity comprises a pairof cavity walls separated by a cavity dimension.
 8. The device of claim7 wherein the cavity walls comprise two mirrors.
 9. The device of claim8 wherein one of the mirrors comprises a broadband mirror and the otherof the mirrors comprises a narrow band mirror.
 10. The device of claim 8wherein both of the mirrors comprise narrow band mirrors.
 11. The deviceof claim 8 wherein both of the mirrors comprise broad band, non-metallicmirrors.
 12. The device of claim 6 wherein the cavity dimension rendersthe cavity resonant with respect to light of the frequency defined bythe spectral characteristics of the mirrors and intrinsic cavity spacingin an undeformed state.
 13. The device of claim 7 wherein one of themirrors comprises a hybrid filter.
 14. The device of claim 7 wherein oneof the walls comprises a dielectric material, a metallic material, or acomposite dielectric/metallic material.
 15. The device of claim 7wherein the cavity is deformable by virtue of a wall that is undertensile stress.
 16. The device of claim 1 wherein the control circuitryis connected for analog control of the impedance to light of eachelement.
 17. The device of claim 16 wherein each modulation elementcomprises an interference cavity having a mechanism for varying thecavity dimension.
 18. The device of claim 17 wherein the mechanismcomprises a deformable wall of the cavity and the control circuitrycontrols the degree of deformation of the cavity.
 19. A device formodulating light in the visible spectrum comprising an array ofmodulation elements, and control circuitry connected to the array forcontrolling the amplitude of light delivered by each of the modulationelements independently by pulse code modulation.
 20. A device formodulation light in the visible spectrum comprising a modulation elementhaving a deformable portion, held under tensile stress, and controlcircuitry connected to control the deformation of the deformableportion. 21-53. (canceled)